The present invention relates to a seat belt tensioning device of a piston-cylinder type.
The seat belt tensioning device of this type is provided with a cylinder, a piston which is disposed within the cylinder and is connected to a seat belt outside of the cylinder through a rod, and a piston operating means which operates at an urgent time such as upon a vehicle impact to move the piston in one direction.
At an urgent time, the piston operating means operates to move the piston so as to tension the seat belt. As a result, the occupant or passenger is restrained on his seat.
The piston operating means could have applied gas pressure, spring force or the like to the piston to move the piston at an urgent time.
When the occupant does not wear the seat belt to which the piston of the seat belt tensioning device of the above type is connected, or he wears the seat belt with an excessively large slack, the piston is moved to its full stroke by the operation of the piston operating means at an urgent time since no tension force is exerted on the seat belt.
As a result, there is a fear that the piston collides against the end wall of the cylinder. By increasing the wall thickness of the cylinder, the damage of the device due to the above collision can be prevented. But, when the wall thickness of the cylinder is increased, the size and the weight of the device becomes large.
Accordingly, one object of the present invention is to provide a seat belt tensioning device of a piston-clyinder type, of which piston does not damage the cylinder even when the piston moves to its full stroke and collides against the end of the cylinder.
Another object of the present invention is to provide a seat belt tensioning device of a piston-cylinder type, which is provided with a means of absorbing piston operating energy when the piston moves by a predetermined stroke.